#4028: Derived Data instance requires Data instances for unused type parameters
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Reporter: igloo | Owner:
Type: bug | Status: new
Priority: normal | Milestone: Not GHC
Component: Compiler (Type checker) | Version: 6.12.2
Keywords: | Difficulty:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Changes (by simonpj):
* cc: leat...@…, dreixel (added)
* milestone: 6.14.1 => Not GHC
Comment:
I've had a chance to look at this. Sadly, 6.10 is wrong here; the change
was the fix to #3087. The derived instance looks like this, as you'll see
if you do -ddump-deriv:
{{{
instance (Data s, Data a) => Data (FSVec s a) where
dataCast2 f = Data.Typeable.gcast2 f -- <------- NOTA
BENE
dataTypeOf _ = tFSVec
toConstr (FSVec _) = cFSVec
gunfold k z _ = k (z FSVec)
gfoldl k z (FSVec a1) = (z FSVec `k` a1)
cFSVec = Data.Data.mkConstr tFSVec "FSVec" ["unFSVec"] Data.Data.Prefix
tFSVec = Data.Data.mkDataType "T4028.FSVec" [cFSVec]
}}}
The new bit, compared to 6.10, is the definition of `dataCast2`. It is
defined in the Data class thus
{{{
class Data a where
dataCast2 :: Typeable2 t
=> (forall d e. (Data d, Data e) => c (t d e))
-> Maybe (c a)
}}}
and `gcast2` has this type:
{{{
gcast2 :: forall t,t',a,b. (Typeable2 t, Typeable2 t')
=> c (t a b) -> Maybe (c (t' a b))
}}}
Now to typecheck the instance
{{{
instance (Data a, Data s) => Data (FSVec s a) where
dataCast2 f = gcast2 f
}}}
we need that `(Data a, Data s)` context. Try that exact instance
declaration and you'll see. And there you are.
If you want to know about `dataCast2` it's all in Section 7 of the SYB2
paper http://research.microsoft.com/en-
us/um/people/simonpj/papers/hmap/gmap2.ps.
If you don't need `dataCast2` you can define it to return `Nothing`:
{{{
dataCast2 f = Nothing
}}}
but you can't use `deriving` for that; you'd have to write out the derived
instance yourself.
I'm not sure how to help you here. I can see that since 's' is phantom it
seems odd to require `(Data s)` but that's what the type checker will
require. My brain has forgotten the details of `dataCast2` and friends.
Maybe some generic programming experts (at Utrecht, for example) might
help? I'm adding Sean Leather to the cc list.
I'll leave this open, but not because there is a bug as specified; rather
maybe there is a better design of the Data library waiting to be
discovered. I'll milestone it as 'Not GHC' though.
I suppose another design would be for derived 'Data' not to define
`dataCast1,2`, but #3087 specifically asked for them to be so!
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4028#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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